1. Field of the invention
The present invention concerns an optical fiber cable and a device for manufacturing a cable of this kind. To be more precise, it concerns an optical fiber cable of the type comprising a stacked plurality of optical fiber ribbons. An optical fiber ribbon usually comprises a plurality of optical fibers disposed side by side in substantially the same plane and held together by means of a common matrix in which they are embedded.
2. Description of the prior art
Such cables are described in patent U.S. Pat. No. 4,078,853, for example. They conventionally comprise:
a stacked plurality of optical fiber ribbons twisted helically upon itself, PA1 an internal protection tube containing the stacked ribbons and disposed loose therein, i.e. with a large radial clearance, the tube being intended in particular to protect the fibers against external radial and longitudinal loads to which the cable may be subjected, possibly to provide a heat barrier to protect the fibers against excessive heat during manufacture of the cable, and to oppose contraction of the outer sheath that may occur during manufacture thereof, in particular, and more generally when the temperature to which the cable is exposed falls; in practise, to protect the optical fibers it is desirable for the resulting contractions of the cable to be restricted to approximately 0.2%, PA1 a protective outer sheath to render the cable resistant to radial compression loads, reinforced with reinforcing members to withstand traction loads on the cable to provide mechanical protection of the optical fibers, the reinforcing members having to this end a modulus of elasticity in traction and an elongation at break greater than those of the optical fibers. PA1 a stacked plurality of optical fiber ribbons each including a plurality of optical fibers disposed side by side in substantially the same plane and all embedded in a common covering or matrix of said ribbon, PA1 a protective outer sheath to protect the cable against radial compression loads, PA1 reinforcing members for withstanding traction loads applied to the cable, which further comprises filler members of a material based on reinforcing fibers and having a coefficient of expansion similar to that of said optical fibers embedded in a connecting matrix, said filler members occupying all or part of a volume delimited by the outside surface of the stacked ribbons and by the inside surface of said outer sheath so that the combination of said stack and said filler members has an external contour in cross-section that is substantially circular, said filler members being adapted to protect said ribbons against radial and longitudinal loads applied to said cable and to oppose contraction of said cable by more than 0.2%.
The stack of ribbons is placed loose in the internal protection tube so that it is decoupled from the latter if the tube is subjected to longitudinal compression loads, for example. The tube is made from a plastics material having a much higher coefficient of expansion than the optical fibers with the result that the performance of the optical fibers can deteriorate in the presence of high longitudinal traction loads. This problem is avoided by decoupling them from the remainder of the cable. In practise the ratio between the cross-section area of the stack of ribbons and that of the volume delimited by the inside contour of the protective tube is between 0.4 and 0.5.
To minimize ingress of moisture into the cable, since it is known that the optical performance of the fibers is very significantly degraded by the presence of moisture, the internal protective tube can contain a waterblocking material such as a thixotropic gel or filaments adapted to swell in the presence of water.
The structure of conventional cables as just described maintains an acceptable level of optical fiber transmission performance, the various component parts of the cable protecting the fibers against the loads to which they are likely to be subjected in use.
The outside diameter of this type of cable (measured across the outer sheath previously mentioned, and bearing in mind that additional sheaths may be used) is usually in the order of 15 mm to 20 mm while the thickness of the stack of ribbons varies between 3 mm and 6 mm.
At present, a constant concern is reducing the overall size of optical fiber cables so that a greater number of cables can be installed in a conduit of given cross-section in order to increase the transmission capacity of networks.
The diameter of cables having the structure described above is too large.
An object of the present invention is therefore to provide an optical fiber cable of the type comprising a stack of optical fiber ribbons having a diameter less than that of prior art cables for a given thickness of the stack of ribbons without compromising the mechanical and optical properties of the prior art cables.